The invention relates generally to methods of fabricating semiconductor devices on silicon wafers, and more particularly to a method of forming a shallow buried insulating layer by implanting molecular ions into a silicon substrate.
Ion implantation is a known technique for processing semiconductor wafers. Particles of material to be implanted are accelerated to high energies and enter the body of a semiconductor wafer, coming to rest at a predictable depth. One use of ion implantation is to form a buried insulation layer within the body of a silicon wafer to electrically isolate a surface layer of silicon, where semiconductor devices are fabricated, from the bulk portion of the wafer beneath the insulation layer.
SIMOX (Separation by IMplanted OXygen) is an ion implantation process in which oxygen ions are implanted into a monocrystalline silicon substrate to form a buried layer of silicon dioxide (SiO.sub.2). The silicon dioxide serves as an insulation layer in the resulting device. A typical SIMOX fabrication process implants atomic oxygen ions (O+) in a substrate using a selected implant energy which can vary between 20 keV to 900 keV, depending on the implantation depth desired. After the atomic oxygen ions (O+) are implanted, the substrate is annealed at a temperature of between 1150.degree. C. to 1400.degree. C. Annealing helps distribute the implanted oxygen ions among neighboring silicon atoms, which tends to sharpen the demarcation between the layers. It also helps repair any damage to the superficial silicon layer caused by the implantation process. The implanted oxygen ions bond with the silicon in the substrate to form a buried layer of silicon dioxide (SiO.sub.2).
SIMOX is a costly and time-consuming process requiring up to approximately twelve hours to implant the oxygen ions, followed by an annealing process which typically takes 6 hours or more. Nevertheless, because SIMOX produces wafers with useful characteristics it remains the subject of ongoing research.
One area of SIMOX research is the implantation of molecular oxygen ions (O.sub.2 +) into the silicon substrate, instead of atomic oxygen ions (O+). Implantation of molecular oxygen ions (O.sub.2 +) has the advantage of doubling the oxygen dose for each electrical charge implanted. It also theoretically reduces the implant time, for a given ion beam current, to half the time required to implant individual atomic oxygen ions (O+). However, the implantation of molecular oxygen ions (O.sub.2 +) has not heretofore been successfully implemented, in part, because prior art processing methodologies caused heavy damage to the superficial silicon, and excessive sputtering. See Ishikawa and Shibata, "Preparation of Thin Silicon-on-Insulator Films by Low Energy Oxygen Ion Implantation," Japanese Journal of Applied Physics, Vol. 30, No. 10, October, 1991, p. 2430. According to Ishikawa et al, implantation of molecular oxygen ions (O.sub.2 +) inflicted great damage to the silicon crystal and resulted in a poor implantation profile due to the reduced projection range of molecular oxygen ions (O.sub.2 +). It also produced an excessive sputtering yield, three times as large as for atomic ion implantation.
Greater success in implanting molecular oxygen ions was reported by Lam and Pinizzotto in "Silicon-On-Insulator By Oxygen Ion Implantation," Journal of Crystal Growth, Vol. 63, 1993, pp. 554-558. Lam et al. reported success in implanting molecular oxygen ions in a silicon substrate using a beam energy of 300 keV and a dose of 1.3.times.10.sup.18 molecular ions/cm.sup.2. The process produced a buried layer of silicon dioxide approximately 5,000 .ANG. thick. While such results demonstrate the feasibility of conventional SIMOX processing using molecular oxygen ions accelerated to high beam energies, there remains a need for improved techniques suitable for the manufacture of shallow SIMOX wafers.
Shallow SIMOX is a variation of conventional SIMOX processing in which the surface silicon and buried insulation layers are made as thin as practicable. The shallow SIMOX process is of increasing importance in fabricating certain highly efficient switching devices such as fully depleted FETs ("Field Effect Transistors"). Fully depleted FETs have a shallow channel region which becomes fully depleted of electrons (assuming N-type conductivity) and develops low-resistance inverse conductivity with a small change in the gate voltage. The shallow channel region is produced by fabricating the device on a wafer with a thin layer of superficial silicon overlying the buried oxide. Shallow SIMOX preferably has an insulating layer which is buried less than 1,000 .ANG. beneath the surface of the substrate, leaving a thin surface layer of monocrystalline silicon in which active devices are formed. It is possible to prepare conventional SIMOX wafers for use in manufacturing fully depleted devices by thinning the surface layer after conventional SIMOX processing. The thinning step adds expense to an already expensive process, however. It would be far better to prepare shallow SIMOX wafers by directly implanting the insulating layer to a shallow depth.
Ion implantation to produce a buried insulating layer in silicon can alternatively use nitrogen, or another insulating material, instead of oxygen. To implant atomic nitrogen ions (N+) in a silicon substrate generally requires approximately the same implant energy and implant dose as is used to implant atomic oxygen ions in the SIMOX process. Following nitrogen implantation, the substrate is annealed in accordance with the SIMOX process. The result is a buried insulating layer of silicon nitride (Si.sub.3 N.sub.4) in the silicon substrate. The process of nitrogen implantation to insulate surface and supporting layers of silicon from one another is often referred to by the name SIMNI.
Still another prior art implantation technique for forming a buried insulating layer in a silicon substrate uses a combination of oxygen ions and nitrogen ions to form a buried layer of siliconoxynitride (Si.sub.x N.sub.y O.sub.z). The process is similar to SIMOX except that both oxygen and nitrogen ions are implanted in the substrate. It is referred to by the name SIMON.
Regardless of whether oxygen or nitrogen is implanted to form an insulating layer in a silicon wafer, the preferred prior art methodology has been to implant atomic ions rather than molecular ions. In particular, molecular ion implantation has not heretofore been used to create shallow SIMOX wafers.
It would be advantageous to provide an improved method of forming a buried insulation layer in silicon by implanting molecular ions instead of atomic ions, thereby increasing the yield of implanted material for each implantation which occurs.
It would also be advantageous to reduce the total electrical charge deposited on a silicon wafer when forming a buried insulation layer using ion implantation.
It would also be advantageous to provide a SIMOX methodology for molecular ion implantation which overcomes the disadvantages of prior art molecular ion implantation techniques.
It would also be advantageous to provide an implantation technique for producing shallow SIMOX wafers wherein molecular ions are implanted to shallow depths within a substrate, thereby reducing or eliminating subsequent processing to thin the surface layer of silicon when fabricating fully depleted active devices.
Accordingly, the present invention provides a method of producing an insulating layer in a silicon substrate using molecular ion implantation. Steps in the method of the invention include (a) providing a source of molecular ions of an insulating material; (b) forming a molecular ion beam from ions provided in step (a), the ion beam having an implantation energy in the range of about 40 keV to 120 keV; (c) implanting molecular ions into a silicon substrate through a first surface of the substrate using the ion beam; and (d) annealing the substrate. The result is a semiconductor substrate with a buried layer of insulating material.
In its preferred form, the invention provides a method of producing a shallow SIMOX substrate with a surface layer of monocrystalline silicon that is less than 1,000 .ANG. thick. Molecular ions of oxygen, nitrogen, or another suitable insulating material, are implanted at an energy of between 40 keV and 120 keV, the energy being selected to cause the implanted molecular ions to come to rest within the substrate in a distribution pattern centered at a depth of less than 1,000 .ANG. beneath the first surface of the substrate. The dose of implanted molecular ions is preferably in the range of about 0.6.times.10.sup.17 to 2.5.times.10.sup.17 molecular ions/cm.sup.2. The result is a buried layer of insulating material which is itself less than 1,000 .ANG. in thickness. The implantation step is preferably carried out with the substrate at an ambient temperature of between about 450.degree. C. to 800.degree. C. The final annealing step is preferably carried out at a temperature generally in the range of about 1,100.degree. C. to 1,400.degree. C. in an ambient atmosphere which is predominately nitrogen or argon.
The present invention provides a practical methodology for creating shallow SIMOX wafers using molecular ion implantation.